Avoid malloc in NotFound key status if no message is given.

Summary:
In some places we have NotFound status created with empty message, but it doesn't avoid a malloc. With this patch, the malloc is avoided for that case.

The motivation of it is that I found in db_bench readrandom test when all keys are not existing, about 4% of the total running time is spent on malloc of Status, plus a similar amount of CPU spent on free of them, which is not necessary.

Test Plan: make all check

Reviewers: dhruba, haobo, igor

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14691

db/memtable.cc

@@ -225,7 +225,7 @@ bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
               *s = Status::Corruption("Error: Could not perform merge.");
             }
           } else {
-            *s = Status::NotFound(Slice());
+            *s = Status::NotFound();
           }
           return true;
         }

db/version_set.cc

@@ -545,7 +545,7 @@ void Version::Get(const ReadOptions& options,
         case kFound:
           return;
         case kDeleted:
-          *status = Status::NotFound(Slice());  // Use empty error message for speed
+          *status = Status::NotFound();  // Use empty error message for speed
           return;
         case kCorrupt:
           *status = Status::Corruption("corrupted key for ", user_key);
@@ -570,7 +570,7 @@ void Version::Get(const ReadOptions& options,
                                    user_key);
     }
   } else {
-    *status = Status::NotFound(Slice()); // Use an empty error message for speed
+    *status = Status::NotFound(); // Use an empty error message for speed
   }
 }